A very large number of different forms of swimming pool alarm have been proposed, and a large number of different alarms have actually been marketed. However, to the best of applicant's knowledge, none of these devices has been entirely satisfactory, the major problem being to provide sufficient sensitivity to trigger the alarm whenever it should be triggered without incurring a large number of false alarms. Whilst from the point of view of demonstration, a high sensitivity to water disturbance is impressive, in practical use it is an unmitigated nuisance unless accompanied by some ability to discriminate between disturbances which require investigation, and those which do not, such as wind disturbances, low flying aircraft, and the impact of twigs and other small objects.
Most of the alarms proposed to date have fallen into three main classes, according to the nature of the alarm transducer. In a first class of alarm, exemplified by the alarms of U.S. Pat. Nos. 3,778,803, 3,475,746, 3,683,353 and 2,723,398, the transducer is essentially wave responsive, triggering the alarm a predetermined magnitude. There are various ways in which such devices may be falsely triggered, but their principal failing is that ripples quite large enough to trigger the alarm can readily be generated by wind.
A second class of alarm discussed at column 1, lines 22-36 and 52-60 of U.S. Pat. No. 3,778,803 is triggered by some form of hydrophone. The problem with this type of alarm is that it can be triggered by loud noises external to the pool, such as low flying aircraft.
A third class of alarm has a pressure sensitive transducer exemplified by the alarm of U.S. Pat. No. 2,935,582 which responds to pressure effects in the water. However, this type of alarm again is too readily set off by minor water disturresponse to a small child or domestic pet struggling in the water.
The object of the present invention is to provide a swimming pool alarm which can provide adequate sensitivity combined with a reduced susceptibility to false alarms.